1. Technical Field
The present invention relates to a substrate stacked image sensor wherein a pixel is completed by forming partial configurations of an image sensor pixel on mutually different substrates, respectively, and three-dimensionally stacking and bonding the substrates; and more particularly, to a substrate-stacked image sensor having a dual detection function wherein a first photodiode is disposed on a first substrate, a second photodiode is disposed on a second substrate, and the first photodiode and the second photodiode are coupled to form a complete photodiode as a configuration element of one pixel when the substrates are stacked and coupled to each other, and wherein the substrate-stacked image sensor can be controlled to selectively read or aggregately read signals individually detected by the respective photodiodes, as necessary.
2. Background Art
Background art may be described from two points of view. One is from the viewpoint of stacking of a semiconductor integrated circuit, and the other is from the viewpoint of miniaturization of an image sensor.
Hereinafter, the conventional techniques on stacking of a semiconductor integrated circuit will be described. As semiconductor integrated circuits are continuously miniaturized, packaging techniques for semiconductor integrated circuits have also been continuously developed to satisfy demands for miniaturization and mounting reliability thereof. Recently, various techniques have been developed for substrate stacking with a three-dimensional (3D) structure in which two or more semiconductor chips or semiconductor packages are vertically stacked.
A three-dimensional (3D) structure element using such substrate stacking is manufactured in such a manner that, after substrates are stacked, the stacked substrates are subjected to a thinning process of grinding the rear surface thereof to reduce the thickness thereof, are subjected to the following processes, are subjected to a sawing process, and then are packaged.
There are very many conventional techniques for stacking substrates in various fields. The present applicant has also tried to develop various techniques. For example, a method for achieving more economic manufacture by omitting an etching process after bonding and stacking substrates is filed by the present applicant and assigned in Korean Patent Application No. 2010-0015632 (Feb. 2, 2010).
In addition, a technique for minimizing the misalignment of bonding pads on the respective substrates when the substrates are bonded with each other is filed by the present applicant and assigned in Korean Patent Application No. 2010-0046400 (May 18, 2010).
In addition, a manufacturing method in which pads are more protruded on the respective substrates in order to facilitate bonding when stacking the substrates is also disclosed in Korean Patent Application No. 2010-53959 (Jun. 8, 2010) granted to the present applicant.
Considering the prior art from the viewpoint of miniaturization of an image sensor, with the development of mobile devices such as portable phones, it is necessary for a camera module built therein to have a lower height and for an image sensor included in a camera module to have a higher resolution in order to increase design flexibility in the mobile devices. By such a tendency, the size of pixels in an image sensor has also continuously decreased.
Recently, with the development of the semiconductor integrated circuit technology, pixels have been able to be manufactured to have a size of approximately 1.4 μm×1.4 μm which approximates to the wavelength band of visible light. Accordingly, in the case of the conventional front side illumination (FSI) scheme, a phenomenon in which light incidented from an exterior is not sufficiently concentrated to a photodiode due to obstacle of metal wirings disposed on the pixels significantly occurs. In order to solve such a problem, an image sensor using a back side illumination (BSI) scheme in which a photodiode is disposed as near as possible to an incident direction of light has been highlighted.
FIG. 1 is a view schematically illustrating an image sensor using the BSI scheme described above, wherein four unit pixels constituted by red, green, and blue color filters 11, 21, 31 and 41, and by photodiodes 12, 22, 32 and 42 are shown in three dimensions. FIG. 2 is a view separating and illustrating only a red pixel from among the pixels. It should be noted that FIGS. 1 to 3 show an embodiment for only a color filter part of pixels included in an image sensor and only a photodiode part formed on a semiconductor substrate.
With the continuous development of the semiconductor technology, the pixels in an image sensor using a BSI scheme have been smaller to a degree that the width thereof is 1.1 μm while the depth thereof is 3 to 5 μm, as shown in FIG. 2, which makes it possible to integrate more pixels per unit area. In this case, a signal disturbance phenomenon, which was not serious before, is raised as a new problem.
Such a problem will be described in more detail with reference to FIG. 3, which is a cross-sectional view of two pixels which are successively disposed. In FIG. 3, light incidented through a green color filter 21 generates photoelectrons in a corresponding photodiode 22. Most of the photoelectrons are normally captured in the depletion region (a portion shown as a dotted line in FIG. 3) of the photodiode 22 connected to the green color filter 21, and become useful current components. However, a part of the photoelectrons cross over into a photodiode 12 of an adjacent pixel, wherein as the width of the photodiodes 12 and 22 are narrower, the amount of photoelectrons crossing over into the photodiode 12 increases. Such photoelectrons are signal losses from the viewpoint of the photodiode 22 connected to the green color filter 21, and are unnecessary signals, i.e. color noise, from the viewpoint of the photodiode 12 connected to the red color filter 11. This is called a cross-talk phenomenon. Consequently, in pixels that the width thereof is as narrow as 1.1 μm while the depth thereof reaches 3 to 5 μm, the cross-talk phenomenon becomes serious, so that the advantage of the BSI scheme does not appear any more.
In a state where pixels have a size (i.e. interval) of 1.1 μm, a substrate may be manufactured to be thin to a thickness of a half or less (e.g. from 4 μm to 2 μm) in order to reduce the cross-talk phenomenon. However, in this case, incident light is not sufficiently absorbed by a silicon photodiode, and a transmittance of passing through the photodiode increases. That is to say, the quantum efficiency (QE) decreases to reduce the amplitude of an electric signal. Here, the quantum efficiency (QE) means a ratio between incident light, i.e. incident photons, and electric charges generated/captured therefrom, and is an index representing how much efficiently light signals are converted into electric signals by an image sensor.
Also, in the conventional back side illumination-type image sensor, the thickness thereof may be reduced in order to reduce the cross-talk phenomenon. However, in this case, it has been well known that: green light is partially absorbed by the photodiode of a first substrate although blue light is mostly absorbed by the photodiode; and red light is absorbed by a smaller amount than the green light although the red light also is partially absorbed. In addition, infrared light shows a tendency to be absorbed by a smaller amount than the red light.
Consequently, absorbing light means that photons have been converted into electric charges. Thus, the problem that the quantum efficiency (QE) is lower in the order of blue light, green light, red light and infrared light becomes larger in the back side illumination-type image sensor. In addition, the components of non-absorbed light are absorbed by other parts, except for photodiodes, are scattered after a collision with metal wirings, or are transmitted deep into stacked substrates, so that light is wasted regardless of the quantum efficiency.